Conventionally, in a semiconductor device manufacturing process, there has been employed a plasma etching method for etching a substrate (e.g., a semiconductor wafer) in a processing chamber by using plasma. By way of example, in the semiconductor device manufacturing process, the plasma etching method has been used to form a contact hole in a silicon dioxide film. When forming a contact hole, there has been a demand for a HARC (High Aspect Ratio Contact). However, it has been difficult to form a contact hole having a high aspect ratio while maintaining a vertical sidewall shape thereof.
As for such a plasma etching method, there is known a technique for forming a contact hole while maintaining a vertical sidewall shape thereof by alternately repeating a period of forming a protection film by using plasma of a gas having a high deposition property and a period of etching the substrate by using plasma of a gas having a low deposition property (see, for example, Patent Document 1).
Patent Document 1: Published Japanese Translation of PCT Patent Application No. 2006-523030
As stated above, in the semiconductor device manufacturing process, although a contact hole having a high aspect ratio is required, it has been difficult to form a contact hole having a high aspect ratio by a plasma etching while maintaining a vertical sidewall shape thereof.
Further, the present inventors have found out that the following problems are generated when a contact hole having a high aspect ratio is formed. That is, if an overetching process is performed after a main etching process under the same conduction for the main etching process suitable for forming a contact hole having a high aspect ratio, a minimum bar (i.e., a thickness of a thinnest portion of a partition wall between adjacent holes) is found to be decreased rapidly in the overetching process (around an etching time of about 680 seconds) as compared to the main etching process, as shown in a graph of FIG. 8. In the graph, a vertical axis represents the minimum bar and a hole depth, and a horizontal axis represents an etching time. In order to prevent the rapid decrease of the minimum bar, if the overetching process is performed under an etching condition where a greater amount of deposits is generated, since a top opening diameter (Top CD) of a hole is small, the hole may be covered with the deposits.